This invention relates to a method of copolymer preparation by solid state polymerization. More particularly the method relates to the preparation of block copolymers incorporating polycarbonate and polyester structural units by solid state polymerization.
Block copolymers are prized for the degree to which polymer physical properties are determined by polymer structure. Block copolymer structure may be varied by adjusting the monomers constituting the blocks, the length of the blocks, and the number of blocks per copolymer molecule. Block copolymers prepared from two different difunctional, mutually reactive oligomers, for example an oligomeric diacid chloride and an oligomeric diol, are referred to as multiblock copolymers and possess a structure in which there are multiple blocks comprising the structural units of the first oligomer alternating with blocks comprising the structural units of the second oligomer. The physical properties of multiblock copolymers may be adjusted through careful control of the block length of the starting oligomers and the choice of a synthetic method which preserves the block length of the starting oligomers in the final multiblock copolymer.
Block copolymers incorporating polycarbonate and polyester structural units, block copolyestercarbonates, have demonstrated effectiveness as UV resistant thermoplastics and hold promise as “weatherable” plastic materials for use in applications in which resistance to the elements is required. Block copolyestercarbonates are typically prepared by reaction of at least one aromatic dihydroxy compound with at least one aromatic dicarboxylic acid dichloride in the presence of water and a solvent such as methylene chloride, an acid acceptor such as sodium hydroxide, and an amine catalyst such as triethylamine to produce a hydroxy-terminated oligomeric polyester. The hydroxy-terminated oligomeric polyester is then further reacted with an additional source of carbonate units, for example phosgene, under interfacial conditions analogous to those used in making polycarbonates such as bisphenol A polycarbonate, in the presence of at least one dihydroxy aromatic compound. A block copolyestercarbonate having polyester blocks and polycarbonate blocks is produced.
Current methods of preparing block copolyestercarbonates suffer from the disadvantages inherent in the use of highly toxic chemicals such as phosgene. Moreover, the use of one or more solvents during the preparation of the hydroxy-terminated oligomeric polyester and during the reaction of the hydroxy-terminated oligomeric polyester with phosgene and at least one dihydroxy aromatic compound requires that steps must be taken to prevent the escape of process solvents from the equipment used. Control measures taken to prevent the escape of process solvents add to the cost and complexity of the manufacturing process. It would be desirable to provide a method for making copolyestercarbonates which did not rely on phosgene and minimized the use of organic solvents.
An alternative methodology, analogous to the melt preparation of polycarbonates, is inapplicable to the manufacture of block copolyestercarbonates due to the tendency of the structural units to randomize under the reaction conditions. Thus, treatment of a mixture of one or more dihydroxy aromatic compounds with a source of ester units such as diphenyl terephthalate, and a source of carbonate units such as diphenyl carbonate in the melt at high temperature in the presence of a catalyst such as sodium hydroxide affords a random copolyestercarbonate owing to the tendency of the structural units of the copolyestercarbonate to achieve a statistical distribution throughout the polymer chains under melt polymerization conditions.
Attempts to incorporate hydroxy-terminated oligomeric polyester intact into polycarbonate chains by reaction of said hydroxy-terminated oligomeric polyester with a source of carbonate units such as diphenyl carbonate and a dihydroxy aromatic compound under the conditions used to prepare melt polycarbonate likewise affords a random copolyestercarbonate owing to the tendency of the polyester blocks to randomize as the polymerization proceeds. In addition to affording random copolyestercarbonates, the “melt” method, although obviating the need for phosgene or an organic solvent such as methylene chloride, requires high temperatures and relatively long reaction times. As a result, by-products may be formed at high temperature, such as the products arising by Fries rearrangement of carbonate and ester units along the growing polymer chains. Fries rearrangement gives rise to uncontrolled polymer branching which may negatively impact the flow properties and performance of the polymer. Moreover, Fries rearrangement may result in “yellowing” of the product copolyestercarbonate. It would be desirable therefore to provide a method for making block copolymers incorporating polycarbonate and polyester structural units which did not require the use of high temperatures and which minimized the formation of Fries product.
Polycarbonates and copolyestercarbonates have been prepared by solid state polymerization (SSP). SSP offers several advantages over both melt phase and the interfacial polycondensation processes. SSP does not require the use of phosgene gas which forms an important element of the interfacial process. Additionally SSP utilizes considerably lower temperatures than those required for the preparation of high molecular weight polycarbonate by melt polymerization of a diaryl carbonate such as diphenyl carbonate and a bisphenol such as bisphenol A. Also, the SSP process, unlike the melt phase process, does not require handling highly viscous polymer melt at high temperatures and the special equipment capable of mixing polymer melt under vacuum at high temperature required in the melt process is not required to perform the SSP process.
In a solid state polymerization process, a precursor polycarbonate, typically a relatively low molecular weight oligomeric polycarbonate, is prepared by the melt reaction of a diaryl carbonate such as diphenyl carbonate with a bisphenol such as bisphenol A. In the preparation of bisphenol A polycarbonate oligomers, a diaryl carbonate such as diphenyl carbonate is heated together with bisphenol A in the presence of a catalyst such as sodium hydroxide while removing phenol. Phenol is formed as a by-product of the transesterification reaction between phenolic groups of the growing polymer chains and diphenyl carbonate or phenyl carbonate polymer chain endgroups. This oligomerization reaction is typically carried out under reduced pressure to facilitate the orderly removal of the phenol by-product. When the desired level of oligomerization has been achieved the reaction is terminated and the product oligomeric polycarbonate is isolated. The oligomeric polycarbonate so produced is amorphous and must be partially crystallized in order to be suitable for solid state polymerization.
The oligomeric polycarbonate may be partially crystallized by one of several methods, such as exposure of powdered or pelletized oligomer to hot solvent vapors, or dissolution of the amorphous oligomer in a solvent such as methylene chloride and thereafter adding a solvent such as methanol or ethyl acetate to precipitate crystalline oligomeric polycarbonate. Typically, such solvent vapor or liquid solvent crystallization methods result in partially crystalline oligomeric polycarbonates having a percent crystallinity between about 20 and about 40 percent as measured by differential scanning calorimetry. A percent crystallinity in this range is usually sufficient for the oligomeric polycarbonate to undergo solid state polymerization without fusion of the pellets or powder being subjected to SSP. In addition to solvent induced crystallization, oligomeric bisphenol A polycarbonate has been crystallized by dissolving diphenyl carbonate in molten amorphous polycarbonate oligomer followed by cooling the mixture to ambient temperature to afford partially crystalline polycarbonate as a mixture with diphenyl carbonate. Finally, amorphous oligomeric polycarbonates have been crystallized by prolonged heating at a temperature below the melting point of the partially crystalline polycarbonate. However, such thermally induced crystallization is quite slow relative to the aforementioned crystallization methods.
The partially crystalline oligomeric polycarbonate in a solid form such as a powder, particulate or pellet is then heated under solid state polymerization conditions at a temperature below the sticking temperature or melting point of the oligomeric polycarbonate, but above the glass transition temperature of the partially crystalline oligomeric polycarbonate, and the volatile by-products formed as chain growth occurs, phenol, diphenyl carbonate and the like, are removed. The polycondensation reaction which converts the low molecular weight oligomer to high polymer is effected in the solid state under these conditions.
Although modern solid state polymerization methods provide a valuable alternative to the melt and interfacial copolyestercarbonate syntheses, the solid state polymerization method suffers from several disadvantages. Typically, the partially crystalline precursor polycarbonate and a partially crystalline oligomeric polyester precursor require two steps for their preparation; an oligomerization step and a crystallization step. Moreover, the solid state polymerization process itself is relatively slow, and affords a random distribution of ester and carbonate structural units within the product copolyestercarbonate. Thus it would be highly desirable to discover improvements which provide greater efficiency in the preparation of the partially crystalline precursor polycarbonate, employ either an amorphous or crystalline oligomeric polyester precursor, and enhance the rates of solid state polymerization such that polymer chain growth proceeds faster than the processes responsible for randomization of ester and carbonate structural units. Such randomization processes typically are manifested by a dramatic reduction in polyester and polycarbonate block lengths.